Reading an Ordnance Survey map

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A hike actually begins before making the slightest step, when the route is worked out on the map. The ability to read a map properly is extremely useful in providing you with all you need to know about what to expect on the hike.

The level of difficulty:

It shows the terrain on a horizontal plane as seen from above, in the form of a simplified diagram. Some training is therefore required in order to see the connection between the map and the terrain.

The scale:

For a hike, the ideal scale is 1:25,000, i.e. a map in which 1 cm is equivalent to 250 m or 4 cm is equivalent to 1 km. It can be used to view each detail (watercourse, forests, paths, etc.) and to accurately work out where you are.

The type of terrain:

The colours used on the map are important as they identify the type of terrain.

- green is used for the woodland areas and white is for the open fields and cultivated areas

- blue is for the hydrography (lakes, rivers, etc.)

- orange is for the contours (see below)

- black is frequently used to identify all the human constructions (highways, constructions, bridges, housing, etc.); it also identifies rocks and scree.

The relief:

The shaded areas that cover the slopes or mountain sides give a sense of the relief. By convention, the theoretical shading shown represents the effect of the sun shining from the north-west at an average angle of 45°. Rather than being useful for identifying where you are, it is simply a tool that makes it easier to identify the slopes when reading the map.

The contours provide additional information about the differences in elevation by defining the volumes and general shapes formed by the terrain. A contour line is an imaginary line that links all the points of equal elevation. On 1:25,000 maps, the contour lines represent elevations in 10m steps. So, the closer the lines are to each other, the steeper the incline. Conversely, spaced-out contour lines mean that the relief is less hilly and gradients gradual.

The notable features:

Details that can often be used to work out exactly where you are. The map publisher takes great care to include all the notable features which can be found on the sites (a spring, a chapel, a cross, a walkway, a ruin, an isolated tree, etc.) The map's legend will help you easily identify the element in question.

Pointing the map in the right direction without instruments:

By convention, north is located at the top of the map. You can rotate the map held horizontally in front of you so as to align the features of the terrain with the map's symbols and point your map in the right direction.